1. Field of the Invention
The present invention relates generally to an exposure apparatus. In particular, the invention relates to an exposure apparatus used for exposing an object to be processed (substrate), such as a single crystal substrate for a semiconductor device or a glass substrate for a liquid crystal display (LCD). The present invention is suitable, for example, for an exposure apparatus using a light beam having a wavelength not longer than 200 nm.
2. Related Background Art
There is an increasing demand for miniaturization of a semiconductor element installed in an electronic device in order to respond to recent requests for reduction in size and thickness of the electronic device. Up to now, a projection exposure apparatus has been used in a lithography (printing) process for manufacturing a semiconductor element, the apparatus projecting and transferring a circuit pattern drawn on a reticle or mask (in this application, the two terms are interchangeably used) onto a wafer, etc., by using a projection optical system.
A resolution (minimum transferable size) R of a projection exposure apparatus is represented by the following expression using a wavelength λ of a light source and a numerical aperture (NA) of a projection optical system:R=k1×λ/NA where k1 represents a process constant defined by a development process, etc.
Accordingly, the shorter the wavelength, the higher the resolution. Based on this, in recent years, an exposure light source has shifted from a conventional extra-high pressure mercury lamp (g line (wavelength: about 436 nm) or i line (wavelength: about 365 nm)) to a KrF excimer laser (wavelength: about 248 nm) or an ArF excimer laser (wavelength: about 193 nm) having a shorter wavelength than conventional ones. Further, an F2 laser (wavelength: about 157 nm) is coming into practical use.
Also, there arises a need to improve a throughput (the number of wafers to be processed per unit time) in a projection exposure apparatus. Since an exposure time for each object to be processed needs to be shortened in order to improve the throughput, an illumination of exposure light, i.e., an exposure amount, in which each object to be processed is irradiated with light per unit time, needs to be increased.
However, an exposure light beam having a shorter wavelength (for example, a light beam having a wavelength in a vacuum ultraviolet region) is absorbed in a larger amount in oxygen or an impurity (water vapor, carbon dioxide, organic material, or halide; also referred to as a contaminant) existing in an atmosphere in an optical path of the exposure light (reduction in transmissivity), resulting in reduction in throughput as well as in exposure light amount of the object to be processed. In general, as the wavelength of the light beam shortens, photon energy gradually increases, causing a photochemical reaction between the contaminant and oxygen. If a product (ammonium sulfate or silicon dioxide) generated by the photochemical reaction adheres to an optical element, the surface of the optical element is fogged, leading to not only a further reduction in exposure light amount, but also, deterioration in imaging performance.
To that end, in a projection exposure apparatus employing a KrF excimer laser or an ArF excimer laser as a light source, optical elements arranged in an optical path are accommodated in a space purged with inert gas in order to avoid reduction in transmissivity due to the absorption of a light beam by oxygen, etc., in an atmosphere in an optical path, or reduction in an exposure light amount and deterioration in imaging performance due to adherence of the product generated by the photochemical reaction to the surface of the optical elements. Also, the purged space is made free of any substance having a gas-producing property, which prevents a contaminant from being generated (see Japanese Patent Application Laid-Open No. H06-029179, for example).
FIG. 9 is a schematic sectional view showing a main structural part of a conventional exposure apparatus 1000. Referring to FIG. 9, the exposure apparatus 1000 includes a lens unit 1200 provided inside a housing 1100, constituting an optical system. An exposure light EL from a light source 1400 is guided to the outside of the housing 1100 through seal glasses 1300a and 1300b. A pipe 1500a for supplying an inert gas and a pipe 1500b for exhausting the air in the housing are provided inside the housing 1100, for example. The inside of the housing 1100 is purged with the inert gas.
The exposure apparatus 1000 includes a rotating member 1700 having a plurality of filters 1600a and 1600b for adjusting a light amount of the exposure light EL, and a motor 1800 for driving the rotating member 1700, inside the housing 1100. The motor 1800 is disposed onto a motor holder 1820 fixed to the housing 1100 through a flange 1810. Note that a motor wiring 1830 is connected to a control unit or a power supply unit through an air-tight connector (not shown).
If the motor 1800 has a gas-producing property, an exhaust gas EG adheres to the lens unit 1200 due to a photochemical reaction, leading to deterioration in imaging performance, as well as a reduction in exposure light amount.
However, with a structure of the aforementioned conventional exposure apparatus, it becomes more difficult to sufficiently suppress a reduction in throughput and deterioration in imaging performance due to contamination of an optical element, as the wavelength of the exposure light becomes shortened. Although the motor is configured so as to minimize the exhaust gas production as mentioned above, it is impossible to use undesirable materials to maintain performance of the motor. For example, a solder or resin is used for a stator or a coil wire wound inside the stator, or other such internal members. Even if materials having a low gas-producing property are basically selected, however, an adhesive, grease, or the like, is used therefor, as well. As a result, as photon energy of the exposure light increases, a photochemical reaction is activated, so that an exhaust gas in a trace amount, which otherwise might cause no photochemical reaction, turns into a contaminant.
The exhaust gas discharged from the motor is known to contain moisture. In particular, a light beam of an F2 laser is largely absorbed by moisture or oxygen. Hence, it is necessary to keep a water and an oxygen content in an optical path at an extremely low level. However, the structure of the conventional exposure apparatus requires a long time to reach such a water or an oxygen content so as to satisfy a required optical performance. This is supposedly because it takes much time to purge with an inert gas the air having intruded and accumulated in the inside of the motor or in minute gaps of a lens holding mechanism or other such mechanisms. Alternatively, this is supposedly triggered by moisture in an exhaust gas discharged from the motor. In addition, for example, if the optical element is fogged, an exposure apparatus needs to be open (canceling the purge with the inert gas) for replacing the optical element; at this time, moisture in the air may be mixed thereinto, so the apparatus should be stopped until the water or moisture content reaches such a level so as to satisfy the required exposure performance again, leading to a reduction in throughput.